Telescopes

The most basic astronomical instrument is the TELESCOPE. A telescope collects light from and magnifies an astronomical object. Until the end of the 19th century, all observational work in astronomy was based on observations made at the eyepiece of a telescope. Nowadays, even though astronomical research relies on much more sophisticated types of measurements, an introductory course in astronomy should develop familiarity with small telescopes and their use in observing the objects discussed in lecture. The main point of this lab will be to familiarize students with setting up and using a small telescope. A total of 6 telescopes will be made available. Four of the telescopes are REFRACTORS, which use lenses to collect light and form an image. Two of the telescopes are examples of the other major class of telescope, the REFLECTOR, which uses a mirror. Two of the refractors to be used have lenses with diameters of 100 and 120 mm, respectively. The other two refractors are Rich Field Telescopes with lenses 80mm in diameter and a short FOCAL LENGTH. More details are given in Table 1 found on the next page. Each pair of students will be assigned to one type of telescope. After this exercise, students should be able to set up these telescopes during field trips and make observations of objects discussed in class. The activities to be carried out in this project consist of the following. Students will set up a telescope on the roof of Van Allen Hall and prepare it for observation. 29:50 Astronomy Lab #4 Stars, Galaxies, and the Universe


F 2
All of these rays are from the same point on the object at infinity, and are all parallel (i.e., U = 0) All of these rays will be focused at this point on the secondary focal plane of the lens N Consider an optical system consisting of a single plus lens and an object at infinity: Where will the rays from the tip of the object be focused?

Object at infinity
Parallel rays are always focused at the secondary focal plane. The precise location can be determined with ray tracing via the nodal point.

F 2
All of these rays are from the same point on the object at infinity, and are all parallel (i.e., U = 0) All of these rays will be focused at this point on the secondary focal plane of the lens θ If we drop all the rays except for the one passing through the nodal point, we can appreciate the angular size N θ

Object at infinity
Telescopes Consider an optical system consisting of a single plus lens and an object at infinity: Where will the rays from the tip of the object be focused?
What would happen if we included a minus lens located such that its primary focal point/plane coincided with the secondary focal point/plane of the plus lens?

Telescopes
What would happen if we included a minus lens located such that its primary focal point/plane coincided with the secondary focal point/plane of the plus lens?
There will be a ray that will pass through the nodal point of the minus lens on its way to the focal plane… F 2 θ N F 1 N …and all of the other rays will be bent to the same angle as the nodal ray.

Object at infinity
There will be a ray that will pass through the nodal point of the minus lens on its way to the focal plane…

Telescopes
What would happen if we included a minus lens located such that its primary focal point/plane coincided with the secondary focal point/plane of the plus lens? What if, instead of inserting a minus lens into the system… we inserted a plus lens, located so its primary focal point coincides with the secondary focal point of the plus lens?

Object at infinity
Image at infinity

Telescopes
If we extend this ray, we can determine the location of the virtual image of the tip of the object Tip of image will appear to be here Low plus objective High plus eyepiece What if, instead of inserting a minus lens into the system… we inserted a plus lens, located so its primary focal point coincides with the secondary focal point of the plus lens?

Object at infinity
Image at infinity

Telescopes
If we extend this ray, we can determine the location of the virtual image of the tip of the object

Inverted
Again, note that the system remains afocal ( What if, instead of inserting a minus lens into the system… we inserted a plus lens, located so its primary focal point coincides with the secondary focal point of the plus lens?

Object at infinity
Image at infinity This is the essence of an astronomical telescope What if, instead of inserting a minus lens into the system… we inserted a plus lens, located so its primary focal point coincides with the secondary focal point of the plus lens?

Object at infinity
Image at infinity

Telescopes
To reiterate: Telescopes come in two basic flavors-those with a high plus eyepiece lens, and those with a high minus eyepiece lens.
High-plus-eyepiece telescopes are called astronomical (or Keplerian) telescopes; and high-minus-eyepiece telescopes are called Galilean (or terrestrial) telescopes.

Telescopes
Parallel rays from an object at infinity

Astronomical (Keplerian) telescope
Low plus lens High plus lens To reiterate: Telescopes come in two basic flavors-those with a high plus eyepiece lens, and those with a high minus eyepiece lens.
High-plus-eyepiece telescopes are called astronomical (or Keplerian) telescopes; and high-minus-eyepiece telescopes are called Galilean (or terrestrial) telescopes.

Galilean (terrestrial) telescope
To reiterate: Telescopes come in two basic flavors-those with a high plus eyepiece lens, and those with a high minus eyepiece lens.
High-plus-eyepiece telescopes are called astronomical (or Keplerian) telescopes; and high-minus-eyepiece telescopes are called Galilean (or terrestrial) telescopes.
In a Galilean telescope, the separation is equal to the difference between the focal lengths.
In an astronomical telescope, the separation is equal to the sum of the focal lengths.
For a telescope to function, the primary focal point of the eyepiece must overlap the secondary focal point of the objective. This determines the separation between the two lenses.

Telescopes
Parallel rays to an image at infinity f 1 of the eyepiece In a Galilean telescope, the separation is equal to the difference between the focal lengths.
In an astronomical telescope, the separation is equal to the sum of the focal lengths. For this and other reasons, Galilean scopes tend to be smaller and lighter than astronomical scopes.
For a telescope to function, the primary focal point of the eyepiece must overlap the secondary focal point of the objective. This determines the separation between the two lenses.

Telescopes
Parallel rays to an image at infinity f 1 of the eyepiece In a Galilean telescope, the separation is equal to the difference between the focal lengths.
In an astronomical telescope, the separation is equal to the sum of the focal lengths. For this and other reasons, Galilean scopes tend to be smaller and lighter than astronomical scopes.
For a telescope to function, the primary focal point of the eyepiece must overlap the secondary focal point of the objective. This determines the separation between the two lenses.
The main other reason being that astronomical telescopes require prisms to flip the inverted image into the upright position, which add considerably to their size and weight!